Vestibular organs, via their hair cells and afferent innervation, transmit information about the direction, speed, and magnitude of head and body movements, which are critical for maintaining our posture and stabilizing our gaze. The vestibular organs of nearly every vertebrate also receive a prominent efferent innervation which begins as a few hundred neurons within the brainstem and extensively collateralizes in the periphery to end as several thousands of bouton terminals on both hair cells and afferents. That this efferent innervation is positioned at such an early stage in the peripheral vestibular pathway suggests it is poised to modulate the initial processing of vestibular cues. Electrical stimulation of efferent neurons results in a diverse panel of profound, yet distinct, excitatory and inhibitory afferent responses where the kinetics of both activation and duration can vary from milliseconds to minutes. To date, three pharmacologically-distinct nicotinic ACh receptors, a muscarinic ACh receptor, at least two classes of potassium channels, and the release of calcium from hair cell internal stores (e.g., subsynaptic cisterns) are thought to underlie the different efferent responses. However, there is a clear gap in our knowledge in relating how and when these different efferent-mediated components impact the responses of vestibular afferents to their natural stimulus. To facilitate an understanding of efferent function in vestibular physiology, three major studies will be performed in the turtle semicircular canal where a strong pharmacological basis for some of the efferent actions has been established, and both hair cell and afferent morphophysiology have been adequately described. The specific aims are: (1) Identify the synaptic mechanisms underlying afferent responses to electrical activation of efferent fibers, (2) Establish how vestibular output is modified by electrical activation of efferent fibers, and (3) Characterize the morphophysiological properties of efferent neurons. To complete specific aims 1 and 2, sharp-electrode and patch clamp recordings will be made from primary afferents, afferent terminals, and hair cells during efferent and mechanical stimulation. Pharmacological agents will be applied to identify the receptors and downstream effectors as well as defining how each efferent synaptic mechanism impacts vestibular stimulation. Parallel immunohistochemical studies will be used to localize the different components implicated by our physiological and pharmacological experiments. For specific aim 3, sharp-electrodes will be used to record efferent activity and label single efferent fibers in a decerebrate preparation. Light and EM microscopy will be used to reconstruct terminal trees and to examine the distribution and synaptic structure of efferent terminals. These studies will provide insights into the mechanisms that the efferent system recruits to modulate afferent discharge as well as identifying synaptic processes that may be targeted pharmacologically for the treatment of diseases and functional defects of the peripheral vestibular apparatus. Given its anatomical prevalence from jawless fish to humans, an efferent innervation of the peripheral vestibular apparatus is necessary for normal vestibular function. Deciphering how and when the vestibular efferent system modifies the initial stages of vestibular processing are crucial to understanding its role in vestibular physiology.